Fluidized-Bed Gasifier
In this design the gasifying agent is blown through a bed of solid particles at a sufficient velocity to keep the particles in a state of suspension. Fuel particles are introduced at the bottom of the reactor and are very quickly mixed with the fluidized-bed material. The fuel is almost instantaneously heated up to the bed temperature by hot solid bed material. As a result of this treatment, the fuel is pyrolyzed very fast, resulting in a component mix with a relatively large amount of gaseous materials. In this type of reactor gasification and tar-conversion reactions occur in the gas phase. All fluidized-bed gasifiers use a bed material, which can be inert sand, the ash from the fuel or catalytic active-bed material like dolomite or olivine. Dolomite is Ca — and Mg-containing mineral rock material composed mainly of CaMg(CO3)2 Olivine is a mineral rock material mainly composed of magnesium iron silicate with the formula (Mg, Fe)2SiO4. The purpose of the bed material is to distribute and transport the heat in the gasifier to prevent local hot spots. The bed material facilitates the mixing of fuel with the gasification gas and promotes the gasification process. Since fluidized-bed gasifiers can handle a wide variety of biomass forms with limited pretreatment, this design is more suitable for large-scale biomass to syngas conversion processes.
Fluidized-bed gasifiers can be further divided into three main categories depending on the actual engineering design. The most common designs are: bubbling fluidized-bed (BFB), circulating
fluidized-bed (CFB) and indirect or allothermal dual fluidized-bed (DFB) types of designs. These designs are shown in Figure 11.3.